here I am again..
This time with a question about nightclub photography.

Nowadays I shoot at (crowded) parties, and I'm finding the 18-105 too 'crappy' for this, I want some nice DoF and better sharpness in my pictures. So I'm planning to upgrade (also because my 50mm 1.8D is too much zoomed for this kind of photography).

First I wanted to buy the 35mm 1.8 almost directly, after reading some good reviews about it. But now I'm thinking it would be also not wide enough for this kinda photography..(?)

Hmm this is a tricky one. If you find the 35mm too much you'll want wider than this. Your kit lens is also f3.5 when this wide, which isnt too far from f2.8 so chances are f2.8 wont be enough if you dont find the kit lens sufficient. There arent many options in this price range for a f2 or faster prime of the required focal length.

Sigma make a 28mm f1.4 I think which might be an option, although its not too much wider than the 35mm

Close quarters are very tough: If you don't have enough space you long for a zoom. And as it's dark you long for a large aperture.
So as f/2.8 zooms are the max you'll get you have to decide what is more important for you:
- dof and subject isolation?
or
- flexibility with "only" f/2.8?

"you can always walk forward" or crop.
A stabilized 24-70/2.8 would be a pretty good instrument for your needs.
But alas, so such lens exists Btw.: A Sigma AF 24mm 1.8 EX DG Asp Macro is not a bad lens (and works on a FX-body too). Price is around 450 EUR (incl. 19% VAT).

Perhaps the Nikon 20mm F2.8 AF or the 24mm F2.8 AF. On a crop frame body that would range anywhere from 30-36mm, which should be good enough. The 28mm F2.8 AF I think would be too long, as it would be about 42mm which would barely fit 4 in the frame.